Power electronics apparatuses such as a switching power supply, an inverter and a lighting circuit of a lighting fixture incorporate a power transformer circuit for transforming power. The power transformer circuit incorporates a switching circuit for transforming a direct current to an alternating current having rectangular waves. Consequently, the power transformer circuit develops a ripple voltage having a frequency equal to the switching frequency of the switching circuit, and noise resulting from the switching operation of the switching circuit. Such a ripple voltage and noise affect other apparatuses. It is therefore required to provide a means for reducing the ripple voltage and noise between the power transformer circuit and the other apparatuses or lines.
LC filters, that is, filters each incorporating an inductance element (an inductor) and a capacitor, are often used as a means for reducing a ripple voltage and noise. The LC filters include a T filter and a π filter, in addition to the one incorporating an inductance element and a capacitor. A typical noise filter for suppressing electromagnetic interference (EMI) is a type of LC filters, too. A typical EMI filter is made up of a combination of discrete elements such as a common mode choke coil, a normal mode choke coil, an X capacitor, and a Y capacitor.
Recently, power line communications have been developed as a potential communications technique used for creating a communications network at home. Through the power line communications, high-frequency signals are superimposed on a power line to perform communications. When the power line communications are performed, noise emerges on the power line because of the operations of various electric and electronic apparatuses connected to the power line, which causes a reduction in quality of communications, such as an increase in error rate. It is therefore required to provide a means for reducing noise on the power line. Moreover, it is required for the power line communications to prevent communications signals on an indoor power line from leaking to an outdoor power line. The LC filters are used as a means for reducing noise on the power line and for preventing communications signals on the indoor power line from leaking to the outdoor power line as thus described, too.
There are two types of noise propagating along two conductor lines: one is normal mode noise that creates a potential difference between the two conductor lines, while the other is common mode noise that propagates along the two conductor lines with identical phases.
FIG. 17 illustrates an example of configuration of an LC filter for reducing common mode noise. The LC filter comprises: a pair of terminals 201a and 201b; another pair of terminals 202a and 202b; a common mode choke coil 203 provided between the terminals 201a, 201b and the terminals 202a, 202b; a capacitor 204 having an end connected to the terminal 201a and the other end grounded; and a capacitor 205 having an end connected to the terminal 201b and the other end grounded.
The common mode choke coil 203 has one magnetic core 203a and two windings 203b and 203c wound around the core 203a. The winding 203b has an end connected to the terminal 201a and the other end connected to the terminal 202a. The winding 203c has an end connected to the terminal 201b and the other end connected to the terminal 202b. The windings 203b and 203c are wound around the core 203a in such directions that, when magnetic fluxes are induced in the core 203a by currents flowing through the windings 203b and 203c when a normal mode current is fed to the windings 203b and 203c, these fluxes are cancelled out by each other.
The LC filter of FIG. 17 is inserted somewhere along the two conductor lines for transmitting power. The terminals 201a and 202a are connected to one of the conductor lines while the terminals 201b and 202b are connected to the other one of the conductor lines.
Next, FIG. 18 illustrates an example of configuration of an LC filter for reducing normal mode noise. The LC filter comprises: a pair of terminals 301a and 301b; another pair of terminals 302a and 302b; a coil 303 as an inductance element; and a capacitor 304. The terminal 302b is connected to the terminal 301b. The coil 303 has an end connected to the terminal 301a and the other end connected to the terminal 302a. The capacitor 304 has an end connected to the other end of the coil 303 and to the terminal 302a. The capacitor 304 has the other end connected to the terminals 301b and 302b. 
The LC filter of FIG. 18 is inserted somewhere along the two conductor lines for transmitting power. The terminals 301a and 302a are connected to one of the conductor lines while the terminals 301b and 302b are connected to the other one of the conductor lines.
The Published Unexamined Japanese Patent Application Heisei 9-102723 (1997) discloses a line filter using a transformer. The line filter comprises the transformer and a filter circuit. The transformer incorporates a secondary winding inserted to one of two conductor lines for transmitting power from an alternating power supply to a load. The filter circuit has two inputs connected to ends of the alternating power supply, and two outputs connected to ends of a primary winding of the transformer. In the line filter, the filter circuit extracts noise components from the supply voltage and supplies the noise components to the primary winding of the transformer, so that the noise components are subtracted from the supply voltage on the conductor line to which the secondary winding of the transformer is inserted. The line filter reduces normal mode noise.
The conventional LC filters have a problem that, since the filters have a specific resonant frequency determined by the inductance and the capacitance, a desired amount of attenuation is obtained only within a narrow frequency range.
For the filter inserted to the conductor line for transmitting power, it is required that desired characteristics be obtained while the current for transmitting power is fed, and that a measure for coping with a rise in temperature be provided. Therefore, such a filter has a problem that the inductance element is increased in size for implementing the desired characteristics.
According to the line filter disclosed in the Published Unexamined Japanese Patent Application Heisei 9-102723, it is theoretically possible to remove noise components completely as long as the impedance of the filter circuit is zero and the coupling coefficient of the transformer is 1. In practice, however, it is impossible that the impedance of the filter circuit is zero, and furthermore, the impedance changes in response to the frequency. In particular, if the filter circuit is made up of a capacitor, a series resonant circuit is made up of the capacitor and the primary winding of the transformer. Therefore, the impedance of the signal path including the capacitor and the primary winding of the transformer is reduced only in a narrow frequency range around the resonant frequency of the series resonant circuit. As a result, the line filter is capable of removing noise components only in a narrow frequency range. In addition, the coupling coefficient of the transformer is smaller than 1 in practice. Therefore, the noise components supplied to the primary winding of the transformer will not be completely subtracted from the supply voltage. Because of these reasons, the line filter actually designed has a problem that it is not capable of effectively reducing noise components in a wide frequency range.